||Checks a session id for validity
||CGITESTSESSION(SessionID : STRING) : INTEGER
||SessionID : a session id returned by CGIGETSESSION, which can be arbitrarily extended.
||0 : session not valid because not matching corresponding IP address
else: 1+number of seconds, that have gone since the session id had been established
A session id should be tested if it is valid espacially at sensible transmission on signals.
CGITESTSESSION does not provide 100 percent protection against vicious attacks, but suspends the huge number of "accidental" accesses to session applied data effeciently.